There are a headache inducing number of threads that never actually resolved this issue on the wizards site. Not linked because I feel dumber after reading them.

Can a hexblade use dual implement spellcaster given that they conjure their weapon from their implement and they have become proficient with their conjured weapon as an implement through multiclassing?

3 Answers
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Dual Implement Spellcaster'sDDI buff requires that you wield a magic implement in each hand, that both implements be usable with the power, and that you must be capable of wielding both implements.

If we assume that by some means light blades have been learned as an implement type (for example via swordmage multiclass) then a fey-pact hexblade could wield their Blade of Winter's MourningDDI as an implement.

Since manifesting the Blade of Winter's Mourning requires that the hexblade hold an implement in one hand, the hexblade now has an implement they are capable of wielding in either hand. And as long as the power being used is (a) an implement power, and (b) not an implement power for another class gained in a way such that the "usable with the power" clause is not violated all the requirements of Dual Implement Spellcaster are met.

The only question remaining is does the Pact Weapon class feature somehow deny this combination. The only relevant passage appears to be:

Based on the information below, which is the basic description of the Level 1 Power: Pact Weapon, I would say that you can use the Dual Implement Feat. However, it would only apply to the powers that include the weapon keyword unless you could figure out a way to have the weapon be considered an implement. I know this isn't exactly supported by the information below but...

My non-rules based opinion is: Why prevent your Hex-Blade from using the defining class feature of the Hex-Blade just because they want to be a dual wielding caster? Sure they save a little $$ by not needing to enchant two implements, however, if you disarm (or steal) the original implement from them they lose access to BOTH. Seems like a potential handicap to me.

Level 1: Pact Weapon

Your pact weapon is a physical manifestation of the arcane alliance you have struck with an otherworldly power.

Benefit: While you hold your implement in one hand, you can use a minor action to manifest your pact weapon in the other hand. The pact weapon you create depends on your pact (fey, page 225, or infernal, page 230). Your pact weapon persists until you no longer hold either it or the implement, or until you dismiss it as a free action.

When you use a power associated with your pact weapon and the power has both the weapon and the implement keyword, you are considered to be wielding both your pact weapon and your implement for the purpose of feats and other game elements.

After being corrected,
I'd say that unless you have a feat or power which explicitly turns a weapon into an implement, it can not be done.

Earlier, I confused the fact that your weapon gains it's powers from your implement, with the weapon being an implement itself. It also states that for hexblade related powers, you are considered to be dual wielding both a weapon and an implement.